This invention relates to a process for reducing the residual acrylonitrile monomer dissolved in acrylonitrile copolymers and in the aqueous media in which acrylonitrile copolymers are polymerized or processed.
Acrylonitrile is employed in the preparation of a wide variety of commercially important resins including for example nitrile rubbers, styrene-acrylonitrile copolymer resins, ABS graft copolymer thermoplastic molding resins and in numerous acrylonitrile-containing graft copolymer impact modifier resins. These copolymers are most often prepared by polymerization in aqueous systems using any of a variety of conventional emulsion and suspension polymerization processes. Even where the polymerization is substantially complete there remains an undesirable amount of unpolymerized acrylonitrile monomer both in the aqueous phase and in the polymer which is removed incompletely and with great difficulty and expense by physical processes such as stripping by steam or vacuum. The presence of residual acrylonitrile in the copolymer is detrimental to the color stability of the resin and may restrict its use due to toxicological considerations and concommitant restrictions imposed by regulation. Similarly, because of environmental considerations the presence of acrylonitrile in the aqueous phase must be minimized where the monomer-containing water will become a part of the plant effluent.
A variety of methods for scavenging residual monomers from the polymer and the aqueous component in combination have been developed in recent years. For example, in U.S. Pat. No. 4,278,582 there is disclosed a process for reducing residual acrylonitrile to levels below 100 ppm by treating the mixtures with ammonium or thio compounds or with a variety of inorganic salts. In U.S. Pat. No. 4,251,412 there is disclosed a method for treating acrylonitrile latices with at least a stoichiometric amount of an alkyl or alkanol amine to reduce the residual monomer content. Although these processes are effective, rather large amounts of the scavenger compound on the order of a stoichiometric (molar) equivalent based on the amount of residual monomer are required. This in turn increases the level of non-aqueous volatiles present in the aqueous phase and may well present further environmental problems.
A process for scavenging the residual acrylonitrile monomer from acrylonitrile copolymers and particularly from both the copolymer and the aqueous phase in a latex which would minimize the amount of scavenger compounds added would thus be a useful advance in the art.